1. Field of the Invention
The present invention relates to a lithographic apparatus and, in particular, to a lithographic alignment system.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, patterning devices, such as a mask, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising part of, one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist).
In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
Prior art lithographic apparatus comprise a conditioned environment, such as a conditioned chamber, of which the interior conditions are controlled. The patterning device is positioned within the conditioned chamber when the desired pattern is projected onto a target portion of the substrate. The conditioned environment is advantageously used when radiation with a short wavelength is used. Such radiation, e.g. extreme ultra violet (EUV) radiation, is easily absorbed by any material and even by air. Therefore, the conditioned environment is an environment with reduced pressure (commonly referred to as vacuum) and/or a reduced particle concentration, i.e. a reduced concentration of dust particles and the like.
In other lithographic apparatus, the conditioned environment may be employed for other reasons than absorption of radiation. For example, the patterning device and/or substrate may be sensitive to gases present in atmospheric conditions. In such a case, only single objects such as the substrate or the patterning device may be positioned in the conditioned environment, in which case the walls of the conditioned chamber need to be transparent to the incident radiation.
In a lithographic apparatus, the patterning device and the substrate may each be replaced to project another pattern onto the substrate, project the same pattern on another substrate, or project another pattern on another substrate. For example, the patterning device may be removed from the conditioned environment and another patterning device may be brought into the conditioned environment. By this operation, the conditioned environment may be exposed at least temporarily to atmospheric conditions and dust, for example. Further, the patterning device and the substrate need to be aligned with a projection beam of radiation to obtain the projection at the target portion of the substrate with the right orientation.
In the prior art lithographic apparatus, a patterning device support structure and/or a substrate table may be positioned within the conditioned environment. The substrate and patterning device are introduced in the lithographic apparatus at a load position outside the conditioned environment. Then, the substrate and patterning device are introduced in the conditioned environment by an actuator moving the substrate and patterning device from the load position to the conditioned environment.
Further, in the prior art lithographic apparatus, an alignment system for aligning the patterning device and/or the substrate is positioned within the conditioned environment. The alignment system comprises at least one position and orientation sensor and at least one actuator. The one or more position and orientation sensors detect the position and orientation of the patterning device and/or the substrate. The actuator, or plurality of actuators, may position the substrate and the patterning device relative to the projection beam of radiation. Then, the projection beam of radiation is imparted with the desired pattern by the patterning device and a target portion of the substrate within the conditioned environment is exposed to the patterned projection beam of radiation.
The alignment system as referred to herein may be an alignment or a pre-alignment system. A pre-alignment system may align an object, such as a substrate or a patterning device, with an accuracy that is lower than the required accuracy. Thereafter, an alignment system may align the object with the required higher accuracy.
The sensor and actuator of the alignment system positioned in the conditioned environment need to be a special, relatively expensive sensor and actuator, since a common, cost-effective sensor and a common, cost-effective actuator may not be resistant to the environmental conditions in the conditioned environment. As an example, vacuum, i.e. a very low pressure, may damage a sensor or an actuator due to pressure differences inside and outside the sensor or actuator. In addition, an actuator may comprise moving parts that may generate particles due to friction between the moving parts, resulting in a pollution of the conditioned environment. Hence, special sensors and actuators are required which can withstand pressure differences, and which do not generate particles.
Not only does the use of the conditioned environment require special sensors and actuators, these sensors and actuators also need to be positioned in a very little space available near the support structures of the substrate and patterning device, resulting in high development and material costs.